1. Field of the Invention
The present invention relates to a receiver for use in a remote control system which makes use of a remote control signal of a predetermined period. More particularly, the invention is concerned with a receiver for use in a wireless remote control system such as a system for controlling the shutter releasing operation of a camera from a remote place.
Hitherto, various types of signal transmitters and receivers suitable for use in remote control systems have been proposed. In general, remote control systems have suffered a problem in that the receiver inevitably receives various noises besides the remote control signal, and the intensity of the remote control signal is lowered substantially in inverse proportion to the square of the distance between the transmitter and the receiver. Therefore, it is a common measure to use a pulse signal as the remote control signal so as to enable the receiver to easily discriminate the control signal, thereby ensuring a high reliability of the remote control despite a large distance between the signal transmitter and receiver.
A single-purpose remote control signal transmitter/receiver, which makes use of pulse-modulated infrared rays as the remote control signal, will be described with specific reference to FIGS. 4 and 5 by way of example.
Referring first to FIG. 4, a transmitter has an oscillation circuit 1 adapted to produce a train of pulses of a predetermined modulating frequency. When a switch 6 is closed, the pulses are delivered to the base of a transistor 3 through a base resistor 5. An infrared light-emitting diode (referred to as "iLED", hereinafter) denoted by a numeral 2 is adapted for emitting an infrared signal. In order to protect the iLED 2 from any overcurrent, a current-limiting protection resistor 4 is connected to the transistor 3. The oscillation circuit 1 and the iLED 2 are activated by power supplied from a source battery 7.
Referring now to FIG. 5, a receiver has a photodiode 8 adapted for receiving the infrared signal, a load resistor 9, a coupling capacitor 10, an amplifier 11, a detector circuit 12 for detecting the signal component of a predetermined signal frequency, a wave shaping circuit 13, and a code conversion circuit 14 which constitutes an interface between the receiver and a camera.
The operation of this remote control system will be explained hereinunder with reference to the waveform chart shown in FIG. 6.
The oscillation circuit 1 shown in FIG. 1 oscillates at the modulation frequency of the remote control signal. The output of this circuit is input to the base of the transistor 3 while the switch 6 is closed as shown by a waveform (a) in FIG. 6. In consequence, the transistor 3 is repeatedly turned on and off at the modulation frequency as shown by a waveform (b) in FIG. 6, so that a pulse current of the modulation frequency is supplied to the iLED 2.
Meanwhile, the photodiode 8 serving as the light-receiving element in the receiver circuit shown in FIG. 5 is negatively biased by the source voltage V.sub.cc through the load resistor 9, so that an electric current of a level substantially proportional to the quantity of the received light flows through the photodiode 8. As a result, a voltage substantially proportional to the received light quantity appears across the resistor 9, i.e., between the ground level and the juncture between the photodiode 8 and the resistor 9. The AC component of this voltage is transmitted to the amplifier 11 through the coupling capacitor 10. This AC component is represented by S; in FIG. 6. The signal S.sub.1 inevitably contains noises, i.e., external lights other than the signal light, such as light from illuminating lamps or fluorescent lamps, as will be seen from the waveform (c) in FIG. 6. The signal S.sub.1 is fed to the detection circuit 12 which picks up only the signal component of the modulation frequency transmitted from the transmitter. Thus, the detection circuit 12 delivers an output signal having a waveform as shown by (d) in FIG. 6. This signal is shaped through the shaping circuit 13 to become a signal S.sub.3 having a waveform as shown in (e) in FIG. 6. This signal S.sub.3 is converted by an interface circuit 14 into a signal for controlling the camera.
Thus, conventional remote control systems employ a pulse-modulated remote control signal which improves the S/N ratio at the receiving side, thereby attaining a higher reliability of the remote control.
In such conventional remote control systems, the detection circuit such as the circuit 12 shown in FIG. 5 usually employs a band-pass filter having a high Q value which represents a sharp frequency selectivity. The use of a band-pass filter having such a high Q value is necessary in order to pick up a weak light signal from among various signal components including noises. The high Q value in turn requires a tuning circuit composed of a coil and a capacitor. The coil, however, is generally large in size compared to other circuit components such as resistors and capacitors and essentially requires means for adjusting the inductance. Consequently the size of the receiver is increased and the cost of the same is raised due to the use of the coil.